An automatic analyzing apparatus which pipettes a reagent and a sample into a reaction container and measures the reactant liquid is equipped with a liquid handler which pipettes reagents and samples into reaction containers.
This liquid handler employs a method of using a pipetting probe made of stainless steel to directly pipette a sample or a reagent, washing the probe which touched a sample or a reagent after pipetting, and using the probe again for next pipetting or a method of applying a disposable tip to a pipetting probe before pipetting and replacing the tip by a new one after pipetting. The method of using a disposable tip can avoid liquid carry-overs between samples, between reagents, and between sample and reagent. As carry-overs between samples cause problems in measurement of immune items which have a wide range of measurement of concentration, the method of using a disposable tip has been frequently used for handling of samples.
However, in pipetting by a tip, the quantity of liquid to be pipetted by the tip may vary as the ambient temperature and relative humidity vary. This is because the saturated vapor pressure of the liquid to be pipetted varies by the ambient temperature and relative humidity and, as the result, the vapor pressure of the gas phase in the tip varies and the quantity of liquid drawn by suction will vary even when the syringe operation is kept constant.
Usually, the tip has a greater contact surface between the liquid to be pipetted and the air in the tip or probe than the stainless steel probe. Therefore, the steam pressure of the air in the tip is greatly dependent upon the vapor of liquid to be drawn by suction and is apt to be affected by the ambient temperature and relative humidity.
As an example of trying to solve the above problem, a method of detecting the level of liquid drawn into the nozzle by suction by a sensor is disclosed by Japanese Patent Application Laid-Open No. 9-288112 (1997).
Judging from the above, it is assumed that the aforesaid problem can be solved by keeping the steam pressure of the air in the tip which draws liquid by suction constant. One of possible methods to solve the problem is to fill the tip with liquid such as water which never reacts with the liquid to be pipetted. Substantially, this method draws a preset amount of water into a tip by suction and drawing a sample liquid into the tip by ejecting the water from the tip.
However, experimentally, we have found that this method cannot draw a sample liquid accurately and repeatedly.